This invention relates to sextants and to celestial navigation systems.
In fixing a position on the earth by celestial navigation, the navigator first uses a sextant to measure the altitude angles above the horizon of at least two celestial bodies, typically stars, planets, and sun and/or the moon. The typical sextant has a frame to which a pivoting tangent arm is attached at one point. A sighting telescope and a horizon glass (on the optical axis of the telescope) are mounted on the frame and a mirror is mounted on the tangent arm at the pivot point. Using the telescope, the navigator sights the horizon through the clear half of the horizon glass, and by pivoting the tangent arm the tangent arm mirror can be caused to reflect onto the mirrored half of the horizon glass an image of a selected celestial body, e.g., a star. An arc surface of the frame is marked to show degrees with respect to a reference line on the tangent arm. When the position of the tangent arm causes the star's image to appear to be at the horizon, the angular reading on the arc tells the altitude angle of the chosen star. Micrometer adjustments of the tangent arm permit accurate readings, with a precision on the order of 1 arc min.